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' S E C O R Conceptual Site Model . Chevron Service Station#9-6171 March 25, 2005 Page 3 2.3 Hydrogeology The majority of the aquifer underlying the site consists of Pliocene and Pleistocene ' continental deposits, Pliocene marine deposits, and Quaternary alluvium The aquifer is characterized as a single heterogeneous body of interbedded fluvial and lacustrine deposits with discrete tenses of gravel, sand, silt, and clay that essentially act as a single semi- confined hydrostratigraphic unit Vertically and horizontally scattered lenses of relatively low permeability provide increasing confinement with depth Groundwater in the San Joaquin Valley typically consists of good quality water that is capped by water of poorer quality and underlain by a body of saline water Overall, groundwater is rich in bicarbonates throughout the Central Valley However, in the San Joaquin Valley, sulfate predominates on the southwest side of the valley and chloride predominates on the northwest side of the valley In the early part of the 20'' Century, the groundwater flow system of the Central Valley was in a state of dynamic equilibrium Recharge to the aquifer occurred via rain and snowmelt in the mountains at the Valley margins, moved downward and laterally Into the aquifer, and then moved upward to discharge at rivers and lakes in the valley axis Therefore, with the exception of the extreme southern portion of the San Joaquin Valley, which is a closed groundwater basin, groundwater flow in the Central Valley ultimately discharges at the San Joaquin-Sacramento River Delta With the construction of hundreds of thousands of wells, groundwater withdrawal jumped from 360,000 acre-feet to 11 5 million acre-feet per year Due to groundwater pumpage the hydraulic heads have been lowered, resulting In the downward movement of water Recharge now occurs from irrigation water, which greatly exceeds the estimated predevelopment recharge rate causing groundwater to flow largely from areas recharged by Imported Irrigation water toward areas of irrigation pumpage In addition, the combination of increased recharge to the water table and increased pumping from the lower zone has caused a reversal In the direction of the hydraulic gradient (from upward to downward) As a result, aquifer storage has decreased by an estimated 60 I million acre-feet This Is due to the dewatering of shallow sediments, inelastic compaction, and a decrease of water table N\FiledProtectslChevron\Retaiil#9-61711Sde Conceptual Model\FINAL_CSM doc